Triaxial force pin sensor array

ABSTRACT

A triaxial force pin sensor array for measuring the actual forces generated in the footprint of a tire includes a high density of individual force pin sensors in a modular design that allows for rapid replacement of defective sensor array elements and substantially prevents dirt and contamination from affecting force measurements. The triaxial force pin sensor array further reduces electromagnetic interference (EMI) and radio frequency (RF) contamination of the sampled data signals. The triaxial force pin sensor array exhibits uniform response in the normal direction. F z , regardless of contact location with individual triaxial force pins of the sensor array. Further, the triaxial force pin sensor array features mechanical overload protection.

TECHNICAL FIELD

This invention relates to an improved sensor array for measuring forces.More particularly, the present invention relates to an improved forcepin, sensor array for measuring triaxial forces, such as those generatedin the footprint of a tire.

BACKGROUND ART

The prior art includes a variety of sensors incorporating strain gaugesbuilt into specialized instruments designed to measure forces on variousobjects. For example, U.S. Pat. No. 2,918,816 discloses an improvedsix-component strain gauge balance system for use in high pressure windtunnels to measure simultaneously the six major forces and momentscorresponding to the six degrees of freedom of any three dimensionalbody under test. The patent describes a cylindrical sleeve for attachingto the tested object and a core within the sleeve for attaching to asupport in the wind tunnel. Annular torsion load cells, axialrhombic-shaped load cells, and diametrical cantilevered load cells areconnected between the sleeve and core to measure the various forces.

In relation to pneumatic tires, European Patent No 0 656 269 A1discloses an essentially two-dimensional array of sensors used toindicate tire inflation by determining the distribution of contactforces over the footprint of a pneumatic tire. The sensors in the arraymeasure only normal force, and are preferably flat detectors such aspiezoelectric or piezoresistive polymer film sensors. A related U.S.Pat. No. 5,396,817 concerns similar measurements utilizing a mainlylinear array of strain gage sensors.

An example of measuring tire forces is seen in U.S. Pat. No. 4,986,118('118) which discloses an array of force sensors, each separatelyconstructed of a vertical hollow tubular member, either square orcylindrical in cross section, with strain gages secured to the verticalsurfaces of the member to measure forces applied by a tire to the topbearing surface of the tubular member. Concentrically inside eachtubular member is a motion sensor to measure relative tire tread motion.The motion sensor includes an elongated pin having a pointed tipextending beyond the bearing surface of the tubular member so that thetip penetrates the tread of the tire under test, and includes straingages secured to the pin to indicate the motion of the pin and thereforethe motion of the tread of the tire penetrated by the pin.

As disclosed in the '118 patent, the prior art includes sensors formeasuring the contact pressure of a tire. For example, one prior artsystem included a plurality of individual pressure sensors in atransducer system to measure the local triaxial contact pressure and thetangential slip pressures in the contact patch, i.e. the “footprint”, ofa tire. This system allowed for measurements along each rib in a treadpattern of a tire to determine factors such as high local pressures andhigh slip pressures that cause uneven tire wear. The prior art sensorswere stable, temperature compensated, high frequency transducers whichwere typically mounted in an array that was strong enough to support amoving tire.

The prior art pressure sensors, as described in a product descriptionentitled “Tire-Road Contact Pressure Sensors” from PRECISION MEASUREMENTCO. of An Arbor, Mich., included individual cantilever pins electricallyconnected to a temperature compensated strain gauge system that enabledeach of the individual pins to simultaneously measure the verticalcontact force, the fore-aft tangential force, and the lateral tangentialforce. A concern relating to the prior art contact pressure sensors wasthe use of a pressure member diaphragm (membrane) at the contact surfacethat was less sensitive at the edges than in the center of the membrane.Also, each of the prior art contact sensor pins was individually mountedwhich, due to geometric size limitations, enabled a limited number ofpins to be joined together for individually measuring the forcesgenerated in the footprint of a tire. In some cases, only a singletriaxial force pin sensor was typically used to fully map the forces andpressures generated in the footprint. To fully map the forces generatedin a tire footprint with the prior art system, the tire would be passedacross a limited number of pressure contact sensors multiple times dueto geometric considerations. The geometric consideration mentioned aboverelates to the minimum center to center distance between adjacentcontact pressure sensors.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a triaxial force pinsensor array, the triaxial force pin sensor array being as defined inone or more of the appended claims and, as such, having the capabilityof being constructed to accomplish one or more of the followingsubsidiary objects.

It is an object of the present invention to provide a triaxial force pinsensor array for measuring triaxial forces, such as those generated inthe footprint of a tire, that obviates the problems and limitations ofthe prior art systems.

It is another object of the present invention to provide a triaxialforce pin sensor array that incorporates a modular design that allowsfor rapid replacement of defective sensor array elements.

Another object of the invention is to provide a method of measuring theactual forces generated in the footprint of a tire with a triaxial forcepin sensor array that substantially prevents dirt and contamination fromaffecting force measurements.

Yet another object of the invention is to provide a triaxial force pinsensor array which reduces electromagnetic interference (EMI) and radiofrequency (RF) contamination of the sampled data signals.

A still further object of the present invention is to provide a triaxialforce pin sensor array which exhibits uniform response in the normaldirection, F_(z), regardless of contact location with individualtriaxial force pins of the sensor array.

Another object of the present invention is to provide a triaxial forcepin sensor array that features mechanical overload protection.

Still another object of the invention is to provide a triaxial force pinsensor array which includes a high density of individual force pinsensors.

Accordingly, there is provided a triaxial force pin sensor array modulethat has two triaxial force pin sensor arrays mounted together. Each ofthe triaxial force pin sensor arrays has a plurality of cantilever forcepins to measure the local normal pressure and the tangential forceapplied to an upper contact surface of the pins. The individualcantilever force pins have a region of reduced cross section extendingaround the circumference thereof and two slots disposed in oppositefacing directions between the region of reduced cross section and thecontact surface. The slots are disposed in spaced relation to the uppercontact surface forming a shear plate or web whose sensitivity to thecontact pressure is determined by the thickness of the web between thereduced section and the diameter of internal bore extending through theweb. Sensors are mounted to the opposite facing side surfaces of thecantilever force pins in the region of reduced cross section and to asurface in between the two slots.

According to the invention, the triaxial force pin sensor array moduleis preferably constructed of a material having a material proportionallimit of at least about 30,000 pounds per square inch (psi) (2.07×10⁸newtons/meter² (n/m²)) and up to about 100,000 psi (6.89×10⁸ n/m²), suchas for example aircraft aluminum or beryllium copper. The high materialproportional limit insures that the force pins will move from the forceapplied thereto without any plastic deformation in the range possibledue to the location of an adjacent pin or side wall of the sensor array.The construction material preferably has natural shielding capabilitywhich reduces electromagnetic interference (EMI) and radio frequency(RF) contamination of the sampled data.

Further in accordance with the invention, a sealant, such as a siliconerubber, having a modulus between about 100 pounds per square inch (psi)(6.89×10⁵ n/m²) and about 1000 psi (6.89×10⁶ n/m²) is disposed abouteach of the cantilever force pins to prevent dust and dirt particlesfrom accumulating therebetween while not materially affecting themovement of the pins.

IN THE DRAWINGS

The structure, operation, and advantages of the presently preferredembodiments of the invention will become further apparent uponconsideration of the following description taken in conjunction with theaccompanying drawings.

FIG. 1 is a three dimensional view of a triaxial force pin sensor arrayin accordance with the present invention;

FIG. 1A is a side view through line 1A—1A of FIG. 1 showing the triaxialforce pin sensor array;

FIG. 2 is a bottom view through line 2—2 of FIG. 1A;

FIG. 3 is a top view through line 3—3 of FIG. 1A;

FIG. 4 is a view through line 4—4 of FIG. 1A;

FIG. 5 is a schematic illustration of a single sensor pin of thetriaxial force pin sensor array of FIG. 1;

FIG. 5A is an enlarged view of the top portion of the triaxial force pinrotated 180° from the position shown in FIG. 5;

FIG. 5B is an enlarged view of the central portion of the triaxial forcepin shown in FIG. 5;

FIG. 6 is an assembly view of two triaxial force pin sensor arraysmounted together to form a triaxial force pin sensor array module;

FIG. 7 is an assembly view of a triaxial fore pin sensor array modulemounted in a frame;

FIG. 8A is an alternative embodiment of a force pin having a radiusedcontact surface;

FIG. 8B is an alternative embodiment of a force pin having a convexedcontact surface; and

FIG. 8C is an alternative embodiment of a force pin having a concavedcontact surface.

FIG. 9 is a schematic illustration of a bridge circuit associated withthe force pin sensors of the triaxial force pin sensor array of thepresent invention for measuring forces, such as those generated in thefootprint of a tire;

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is illustrated a three-dimensional view of atriaxial force pin sensor array 10 in accordance with the presentinvention. The sensor array 10 includes a force pin support structure 12having a first and second opposite side portions 14 and 16, upper andlower end surfaces 18 and 20, and first and second side surfaces 22 and24. At either side of the support structure 12 there is located amounting portion 26 and 28 having a throughbore 30 and 32, respectively,through which an attachment bolt (not shown) secures the sensor array 10to a sensor array mounting frame structure 34 as illustrated in FIG. 7and discussed in detail hereinafter.

CANTILEVER TRIAXIAL FORCE PINS

Preferably, the support structure 12 includes a plurality of cantilevertriaxial force pins 36 a, 36 b, 36 c, 36 d, 36 e, 36 f, 36 g, 36 h(36a-36 h), which are located adjacent to each other and are integrallyattached to the base portion 40 of the support structure 12. Each of thecantilever force pins 36 a-36 h are identical and therefore only pin 36b, as illustrated in FIGS. 4, 5, 5A, and 5B, is described in detailsince the details of the construction of each triaxial pin aresubstantially the same. While eight cantilever force pins 36 a-36 h areillustrated, it is within the terms of the invention to provide more orfewer cantilever force pins depending upon the specific application.

Cantilever pin 36 b has an upper contact surface 42 b which can be flat,radiused, convexed or concaved, as shown in FIGS. 5, 8A 8B, and 8C,respectively. In the preferred embodiment of the invention, the contactsurface 42 b is flat as shown in FIG. 1. The forces being measured,typically those generated in a tire footprint, are transferred to thecontact surface of each of the cantilever pins 36 a-36 h. The contactsurface 42 b (see FIGS. 4 and 5) of cantilever pin 36 b preferably has arectangular cross-section disposed in a plane perpendicular to thelongitudinal axis 44 b extending through the cantilever pin, although itis within the terms of the invention to construct the cantilever pins 36a-36 h with alternate cross-sections, such as for example, circular andoval cross-sections.

Pin 36 b, as shown in FIGS. 4, 5, 5A and 5B, has an upper portion 46 bwith a rectangular or square cross-section that has opposite front andrear surfaces 48 b and 50 b, respectively, spaced a distance w₁ fromeach other and opposite side surfaces 52 b and 54 b spaced a distance t₁from each other. Below the upper portion 46 b is an intermediate portionor region 56 b which also has a substantially rectangular or squarecross-section that has opposite front and rear surfaces 58 b and 60 b,respectively, spaced a distance w₂ between each other and opposite sidesurfaces 62 b and 64 b, respectively, spaced a distance t₂. The upperportion 46 b is contiguous with and integrally attached to theintermediate portion 56 b by a plurality of curved surfaces which extendaround the perimeter of the force pin, i.e., interconnecting surfaces 48b and 58 b, surfaces 50 b and 60 b, surfaces 52 b and 62 b, and surfaces54 b and 64 b, as best seen in FIG. 5A. A feature of the invention isthat the distance w₁ and t₁ between the opposite surfaces in upperportion 46 b are longer, respectively, than the corresponding oppositesurfaces in intermediate portion 56 b having distances w₂ and t₂. Belowthe intermediate portion 56 b is a base portion 66 b which also has asubstantially rectangular or square cross-section and has opposite frontand rear surfaces 72 b and 74 b spaced a distance w₃ from each other andopposite side surfaces 68 b and 70 b, respectively, spaced a distance t₃from each other. The intermediate portion 56 b is contiguous with andintegrally attached to the base portion 66 b by a plurality of curved,concave surfaces, as best seen in FIG. 5. The bottom surface 76 b of pin36 b is contiguous with and forms an integral portion of base portion 40of support structure 12. Since the pins 36 a-36 h are each attachedsoley at the bottom of their intermediate portion, i.e. by the concavesurfaces to the base portion of the support structure 12, the pins areeffectively mounted in a cantilevered manner with respect to baseportion 40.

TRIAXIAL FORCE PIN SENSOR ARRAY

The triaxial force pin sensor array 10, which includes a plurality ofcantilever triaxial force pins 36 a-36 h, can be machined from a singleblock of material into a shape as generally shown in FIGS. 1, 1A, 2 and3. That is, the block of material is initially shaped into a generallyrectangular form having a distance w₄ in the mounting portions 26 and 28and a height h₁ between the upper end surface 18 and the lower endsurface 20, see FIGS. 1 and 1A. A plurality of slits 80 are open at theupper end surface 18 of the support structure 12 and extend to thelocation 81 which coincides with the upper end of elongatedthrough-slots 82 a-82 i between adjacent triaxial force pins 36 a-36 h,as discussed in detail below.

Continuing, a plurality of generally oval shaped channels 82 a, 82 b, 82c, 82 d, 82 e, 82 f, 82 g, 82 h, 82 i (82 a-82 i) of height h₂ extendcompletely through the distance w₄ of the support structure 12 asgenerally shown in FIG. 1A. Each of these channels 82 a-82 i reduces thewidth of pins 36 a-36 h from the initial spacing of t₁, as in the upperportion 46 b, to the spacing t₂ in the intermediate portion 56 b, asshown in FIG. 5. Then the front and rear surfaces 58 b, 60 b,respectively of intermediate portions 56 b of pin 36 b, as well as theremainder of the pins, are formed so that the intermediate portions 56a-56 h are rectangular or square about a longitudinal axis 44 a-44 h(only 44 b being illustrated) extending through the pins. Theintermediate portions 56 a-56 h each have four curved, concave surfacesextending about the upper end and contiguously joined with the upperportions 46 a-46 h of the pins 36 a-36 h, respectively. The intermediateportions also have four curved, concave surfaces extending about thelower end which are joined contiguously with the base portions 66 a -66h of the pins 36 a-36 h, respectively.

A pair of slots 90 a, 90 b, 90 c, 90 d, 90 e, 90 f, 90 g, 90 h (90 a-90h) and 92 a, 92 b, 92 c, 92 d, 92 e, 92 f, 92 g, 92 h (92 a-92 h) withradiused closed ends for each pin 36 a-36 h, as shown in FIGS. 1A, 5 and5A, extend from the front surface 48 b to the rear surface 50 b of theexemplary cantilevered pin 36 b. The upper slots 90 a-90 h are parallelwith their corresponding lower slots 92 a-92 h in each pin 36 a-36 h,respectively. The pair of corresponding upper and lower slots for eachpin open outwards from opposite facing side surfaces, i.e., 48 b and 50b in the example of pin 36 b. A connecting section 87 b, extending adistance h₅ between the upper slot 90 b and the lower slot 92 b, hasfront and rear facing surfaces 91 b and 93 b which are recessed inwardfrom the front and rear surfaces 48 b and 50 b, respectively, of portion46 b and are spaced from each other a distance w₅ which is less thandistance w₁. Preferably, the front and rear facing surfaces 91 b and 93b are each in a plane which is parallel to the plans in which the frontand rear surfaces 48 b and 50 b of upper portion 46 b are disposed. Theorientation of slots in adjacent pins may be mirror imaged to facilitatemachining operations. With this configuration, normal loads (F_(z)) canbe accurately sensed regardless of contact position with the pin 36 b onupper surface 42 b. A plurality of blind bores 108 a-108 h extend upthrough the lower end 20 of the support structure 12 into each of thecantilever force pins 36 a-36 h, respectively. The blind bores 108 a-108h pass through slots 92 a-92 h and open into slots 90 a-90 h to providea passageway to receive electrical signal conducting wires (not shown),as discussed below. Also, a pair of spaced through holes 110 a-110 h arecut through each pin 36 a-36 h from the front side 58 b to the internalbore 108 b. Also a pair of through holes 111 a-111 h are cut from therear side 60 b of the pin and into the internal bore 108 b, as shown inFIG. 4. The sets of spaced through holes 110 a-110 h and 111 a-111 h aredisposed preferably with their longitudinal axis 113, 115 forming anangle “x” of about 90°. The electrical signal conducting wires from thestrain gauges, discussed below, are passed through the holes 110 a-110 hand 111 a-111 h and into their corresponding bores 108 a-108 h.

The material for constructing the support structure 12 is preferably amaterial selected from the group consisting essentially of aircraftaluminum (Al2024) having a material proportional limit of 30,000 psi(2.07×10⁸ newtons/meter² (n/m²)) and beryllium copper (BeCu) having amaterial proportional limit of 100,000 psi. (6.89×10⁸ n/m²). While boththe beryllium copper and the aircraft aluminum are effective for thepresent design of the pin sensor array 10, the beryllium copper is moreadvantageous in that it has a more durable wear surface and a highermaterial proportional limit. The higher material proportional limit isparticularly important because it is desirable that as the force pins 36a-36 h move, they will naturally return to their original positionswithout any plastic deformation. This is particularly the case in theirmovement towards one side 22 or the other side 24 of the supportstructure 12. With this movement, the pins 36 a-36 h will abut eitheragainst an adjacent pin or possibly side portions 14 and 16 in the caseof pins 36 a and 36 h as a pin is moved from the forces applied thereto.Since the material proportional limit of the beryllium copper is high,any contact with an adjacent force pin or side portions 14, 16 willautomatically prevent plastic deformation of the moving pin because thematerial proportional limit of the sensor array 10 requires a greaterdegree of movement to achieve plastic deformation than that afforded bythe distance between adjacent pins and/or the side portions. Anotheradvantage of the beryllium copper is it being a relatively rigidmaterial. A still further advantage of the beryllium copper is itsnatural shielding capability which reduces electromagnetic interference(EMI) and radio frequency (RF) contamination of the sampled data.

Three sets of strain gauges 112, 114, 116, as best shown in FIGS. 5, 5A,and 5B, are mounted onto each cantilever pin 36 a-36 h of the force pinsupport structure 12. For example, the first set of strain gauges 112includes two dual strain gauges 112 a and 112 b which are mounted ontothe side faces of reduced sections 91 b and 93 b, respectively, as shownin FIGS. 5 and 5A. The second set of strain gauges 114 as shown in FIG.5B, which includes four strain gauges 114 a, 114 b, 114 c, 114 d aremounted on opposite sides 58 b and 60 b, respectively, of the pin 36 bin the intermediate portion 56 b. The third set of strain gauges 116,which includes four gauges 116 a, 116 b, 116 c and 116 d, are located onthe sides 62 b and 64 b, respectively, of intermediate portion 56 b.Each of the strain gauge sets 112, 114, and 116 has signal conductingwires (not shown) which, in the case of strain pair 112 are directedthrough the bore 108 b and out the lower open end for attachment to acircuit card 120 as described below. The strain gauge sets 114 and 116have signal conducting wires disposed through bores 110 b and 111 bsignal conducting wires disposed through bores 110 b and 111 b whichintersect bore 108 b so that the wires can also project outward from theopen end of bore 108 b, as shown in FIG. 2. These signal conductingwires, which extend outwards from the lower end 20 of the supportstructure 12, can be attached to a circuit card 120 incorporating aplurality of copper strips. Two of the triaxial force pin sensor arrays10 are mounted to opposite surfaces of the circuit card 120, as shown inFIG. 6, by conventional means such as screws. This combination of twotriaxial force pin sensor arrays with a circuit card therebetween,constitutes a single replaceable force sensor array module 130.

Because of the differences in operating environments, the strain gaugesrequire proper thermal characteristics. Generally, strain gauges can beselected with thermal compensation for steel, copper or aluminum asdetermined by the material to which the gauges will be attached. Sincethe material from which the cantilever pins 36 a-36 h are constructed ispreferably beryllium copper, copper compensated gauges will preferablybe selected for the sets of strain gauges 112, 114 and 116. The gaugescan be mounted by conventional means, such as an epoxy adhesive, to thepins.

PROTECTIVE SEALANT

Around each of the cantilever pins 36 a-36 h, including within the slits80, is provided a low modulus sealant to prevent dust and dirt particlesfrom accumulating therein. The sealant has a modulus of between about100 psi (6.89×10⁵ n/m²) and 1000 psi (6.89×10⁶ n/m²). It is believedthat a modulus below about 100 psi, which would typically be a gel,would lack the adhesion needed to be utilized for the purpose ofpreventing the accumulation of dirt and dust particles. On the otherhand, if the modulus became too high, such as above about 1000 psi, thematerial would prevent the free movement of the pins as needed. Apreferred material is a silicone rubber RTV No. 3140 from Dow-Corning.

SENSOR ARRAY MODULE

Preferably, a complete triaxial force pin sensor array module 130, asshown in FIG. 6, would include two triaxial force pin sensor arrays 10with a circuit card 120 having copper bars disposed there between. Thesignals from the sets of strain gauges 112, 114 and 116 can be takendirectly from the sensor array modules 130 and then conditioned andprocessed in a data collecting system 132, as shown in FIG. 7, such asfor example a Datronic Model 10K7 measurement and control unit fromDatronic Corporation of Miamisburgh, Ohio. In accordance with theinvention, a single sensor array module 130 can be quickly and easilyreplaced, with a minimum of downtime, when defective. Moreover, thesensor array module 130 can be constructed with cantilever pins in closeproximity in the x—x and y—y directions providing for high densitymeasurements.

An important aspect of the cantilevered pin construction relates to thedimensional relationship within each pin 36 a-36 h. The lateral andtangential (fore-aft) compliance (measure of stiffness) is determined bythe placement of the intermediate portion 56 a-56 h with respect to thebase portion 66 a-66 h, respectively, and the upper portion 46 a-46 h,respectively. For example, as the intermediate portion 56 a-56 h islocated closer to the bottom surface 76 a-76 h, respectively, the gainor total displacement of the upper surface 42 a-42 h increases for agiven load. Moreover, the heights of the intermediate portions 56 a-56 hdetermines the maximum lateral or tangential load that can be appliedbefore mechanical interference with an adjacent pin is reached.

In a like manner, the sensitivity for measurement of normal pressures orforces is governed by the selected width w₅ for shear plate members 87a-87 h in conjunction with the depth of slots 90 a-90 h and 92 a-92 h,respectively. For example, as the width w₅ of shear plate member 87 b,as shown in FIGS. 5 and 5A, is reduced, the response is increased for agiven normal load. The distances w₂, t₂, and h₂ of the intermediateportion 56 a-56 h also affects the compliance or deflection of thecorresponding cantilevered pin 36 a-36 h with a given lateral ortangential load.

The diameter of the blind bore 108 a-108 h also affects the complianceor deflection of its corresponding cantilevered pin. Thus, as thediameter of the bore 108 a-108 h increases, the compliance or deflectionin the lateral, tangential and normal directions increases for a givenload.

In a related aspect of the pin construction, dimensional variations pinto pin will naturally occur. These variations are accommodated bymaintaining individual calibration values for each pin.

STRAIN GAUGE MEASUREMENTS

The strain gauge sets 112, 114, 116 are mounted on pins 36 a-36 h tomeasure the normal and shear forces generated on the upper contactsurfaces 42 a-42 h of the cantilever pins 36 a-36 h, respectively. Foruse with a tire, a plurality of the sensor array modules 130 can bemounted in a sensor array mounting frame structure 34, only a singlemodule 130 being illustrated in FIG. 7, so that the fore-aft direction,i.e. the direction in which the tire being measured rotates across theupper surface of the modules, is in an x—x direction to measure acomplete tread element in a single pass. Transverse to the fore-aftdirection is the lateral or y—y direction. The normal or load contactpressure is measured in the z—z direction. A second embodiment of theinvention provides for rotation of the sensor array mounting framestructure 34 such that the x—x and y—y directions are inverted orreversed to measure an entire rib of a tire in a single pass over thetriaxial force sensor array modules in box-like structure 34.

The upper set of strain gauges 112 a and 112 b measure normal load F_(x)on the shear plate section 87 a-87 h of the pins 36 a-36 h,respectively. The shear plate section 87 a-87 h is generally thatsection between the lower slots 92 a-92 h and the upper slots 90 a-90 hof the pins 36 a-36 h, respectively. As a load, such as a tire, rollsacross the contact surfaces 42 a-42 h of a pin 36 a-36 h, the pindeflects to change the resistance of strain gauge pairs 112 a and 112 bof set 112.

In the same manner, the strain gauge sets 114 and 116 respond tomovements of the upper contact surface 42 a-42 h in combinations offore-aft and/or lateral directions. The signals generated by the straingauge sets 112, 114, 116 are fed to Wheatstone bridges 140, as shown inFIG. 9, with resistors A, B, C, D (represented as A-D), one on eachside. While each of the resistors A-D, in the preferred embodiment, hasa value of about 700 ohms, it is within the terms of the invention toselect resistors of different values. For example, strain gauge pairs112 a and 112 b are each wired in series and each pair has a value ofabout 700 ohms. The Wheatstone bridge 140 is typically excited with avoltage of about 10 volts, however the specific voltage is not a part ofthe present invention and other voltages can be used. As shown in FIG.9, the output signal is measured across points 142 a and 142 b, afterthe Wheatstone bridge has been balanced in a conventional manner. Eachstrain gauge set 112, 114, 116 represent two legs of a Wheatstonebridge, the remaining two legs 700 ohms each (corresponding to theexample of the preferred embodiment), are provided with bridgecompletion circuitry.

It is apparent that there has been provided in accordance with thisinvention a triaxial force pin sensor array that can be used formeasuring the actual forces generated in the footprint of a tire. Thetriaxial force pin sensor array includes a high density of individualforce pin sensors in a modular design that allows for rapid replacementof defective sensor array elements and substantially prevents dirt andcontamination from affecting force measurements. The triaxial force pinsensor array further reduces electromagnetic interference (EMI) andradio frequency (RF) contamination of the sampled data signals. Thetriaxial force pin sensor array exhibits uniform response in the normaldirection, F_(z), regardless of contact location with individualtriaxial force pins of the sensor array. Further, the triaxial force pinsensor array features mechanical overload protection.

While the invention has been described in combination with embodimentsthereof, it is evident that many alternatives, modifications, andvariations will be apparent to those skilled in the art in light of theforegoing teachings. Accordingly, the invention is intended to embraceall such alternatives, modifications and variations as fall within thescope of the appended claims.

We claim:
 1. A triaxial force pin sensor array for performing triaxialforce measurements at multiple adjacent locations, the triaxial forcemeasurements at each location consisting of simultaneous measurements oflinear forces along three orthogonal axes, the triaxial force pin sensorarray characterized by: a plurality of cantilever force pins machinedfrom a single rectangular block of material to form a linear array ofadjacent cantilever force pins integral with a support structure,wherein a side surface of each one of the plurality of cantilever forcepins is facing a side surface of an adjacent one of the plurality ofcantilever force pins; each of the plurality of cantilever force pinsbeing constructed for performing triaxial force measurements, by havingan upper portion with an upper contact surface for sensing normal forceand tangential forces, both fore-aft and lateral, applied to the uppercontact surface; and a base portion integral with a base portion of thesupport structure; each of the plurality of cantilever force pins havingan intermediate portion of reduced cross section between the upperportion and the base portion; the linear array and each of the pluralityof cantilever force pins having a front surface and a back surfaceparallel to the plane of the linear array; and each cantilever force pinhaving two slots machined through from the front surface to the backsurface and disposed across the upper portion of the cantilever forcepin in spaced relation to the upper contact surface to form a shearplate section between the two slots, each of the two slots opening toopposite facing side surfaces of the cantilever force pin and beingdisposed at different distances from the upper contact surface.
 2. Thetriaxial force pin sensor array of claim 1 wherein each cantilever forcepin is characterized in that: the reduced cross section of theintermediate portion extends around the perimeter of the cantileverforce pin so that the intermediate portion is rectangular with respectto an axis extending longitudinally through the cantilever force pin. 3.The triaxial force pin sensor array of claim 2 wherein each cantileverforce pin is characterized by: a first set of sensors mounted to theshear plate section of the upper portion for measurement of normalforces; and a second set and a third set of sensors mounted to theintermediate portion for measurement of fore-aft and lateral tangentialforces, respectively.
 4. The triaxial force pin sensor array f claim 3wherein each cantilever force pin is characterized in that: all sensorsin the first set of sensors, the second set of sensors, and the thirdset of sensors are strain gauges.
 5. The triaxial force pin sensor arrayof claim 4 wherein each cantilever force pin is characterized in that:all of the strain gauges are mounted and wired in a way that measuresorthogonal linear forces while avoiding measurement of moment forcecouples, such that: the first set of sensors comprises two pairs ofstrain gauges with a first pair mounted on the front surface of theshear plate section and a second pair mounted on the back surface of theshear plate section, wherein the first pair is wired in series to form afirst leg of a normal force bridge circuit and the second pair is wiredin series to form a second leg of the normal force bridge circuit; thesecond set of sensors comprises two pairs of strain gauges with a firstpair mounted on the front surface of the intermediate portion and asecond pair mounted on the back surface of the intermediate portion,with the sensors of each pair being located one at the top and one atthe bottom of the intermediate portion, wherein the first pair is wiredin series to form a first leg of a fore-aft force bridge circuit and thesecond pair is wired in series to form a second leg of the fore-aftforce bridge circuit; and the third set of sensors comprises two pairsof strain gauges with a first pair mounted on a first side surface ofthe intermediate portion and a second pair mounted on an opposing secondside surface of the intermediate portion, with the sensors of each pairbeing located one at the top and one at the bottom of the intermediateportion, wherein eh first pair is wired in series to form a first leg ofa lateral force bridge circuit and the second pair is wired in series toform a second leg of the lateral force bridge circuit.
 6. The triaxialforce pin sensor array of claim 2 wherein each cantilever force pin ischaracterized in that: the intermediate portion of the cantilever forcepin has a substantially square cross section in a plane normal to anaxis extending longitudinally through the cantilever force pin.
 7. Thetriaxial force pin sensor array of claim 1 wherein each cantilever forepin is characterized in that: the cantilever force pin has asubstantially square cross section in a plane normal to an axisextending longitudinally through the cantilever force pin.
 8. Thetriaxial force pin sensor array of claim 7 wherein each cantilever forcepin is characterized by: a first set of strain gauges mounted to theshear plate section of the upper portion; a second set and a third setof strain gauges mounted to the intermediate portion; at least one frontthrough hole cut through the intermediate portion from the front surfaceto the circular bore, and at least one back through hole cut through theintermediate portion from the back surface to the circular bore; and thefirst, second and third sets of strain gauges having electrical wiresthat pass through the front through holes or the back through holes andthen through the circular bore to be connected to a circuit card.
 9. Thetriaxial force pin sensor array of claim 1 wherein each cantilever forcepin is characterized by: a circular bore extending through thecantilever force pin from the base portion to the slot closest to theupper contact surface.
 10. The triaxial force pin sensor array of claim1 wherein each cantilever force pin is characterized in that: thecantilever force pin is constructed of a material having a materialproportional limit of at least about 30,000 psi (2.07×10⁸ n/m²) to about100,000 psi (6.89×10⁸ n/m²).
 11. The triaxial force pin sensor array ofclaim 10 wherein each cantilever force pin is characterized in that: thecantilever force pin is constructed of a material selected from thegroup consisting of aircraft aluminum and beryllium copper.
 12. Thetriaxial force pin sensor array of claim 1 wherein each cantilever forcepin is characterized in that: the upper contact surface is flat.
 13. Thetriaxial force pin sensor array of claim 1 wherein each cantilever forcepin is characterized in that: the upper contact surface is concave. 14.The triaxial force pin sensor array of claim 1 wherein each cantileverforce pin is characterized in that: the upper contact surface is convex.15. The triaxial force pin sensor array of claim 1 wherein eachcantilever force pin is characterized in that: the upper contact surfaceis radiused.
 16. The triaxial force pin sensor array of claim 1characterized by: a sealant around each of the plurality of cantileverforce pins, the sealant having a modulus between about 100 psi (6.89×10⁵n/m²) and about 1000 psi (6.89×10⁶ n/m²).
 17. The triaxial force pinsensor array of claim 16 characterized in that: the sealant is siliconerubber.
 18. The triaxial force pin sensor array of claim 1 characterizedby: a triaxial force pin sensor array module comprising two triaxialforce pin sensor arrays mounted together with a circuit card disposedtherebetween to form a rectangular array of cantilever force pins.
 19. Amethod of performing triaxial force measurements at multiple adjacentlocations, the triaxial force measurements at each location consistingof simultaneous measurements of linear forces along three orthogonalaxes, the method characterized by the steps of: utilizing a triaxialforce pin sensor array comprising a plurality of cantilever force pinsmachined from a single rectangular block of material to form a lineararray of adjacent cantilever force pins integral with a supportstructure; measuring forces applied to an upper contact surface on anupper portion of one or more cantilever force pins mounted at anopposite base portion end integral with a base portion of a supportstructure, each of the cantilever force pins having an intermediateportion of reduced cross section between the upper portion and the baseportion and having two slots disposed across the upper portion of thecantilever force pin in spaced relation to the upper contact surface toform a shear plate section; measuring shear plate signals with a firstset of strain gauges mounted to the shear plate section, the shear platesignals corresponding to the normal fore applied to the upper contactsurface; and measuring forces tangential to the upper contact surfaceswith second and third sets of strain gauges mounted to the intermediateportion of the cantilever force pins, the force signals corresponding tothe tangential fores applied in the fore-aft and lateral directions tothe upper contact surface.
 20. The method of claim 19 characterized byfurther including the steps of: conditioning and processing the shearplate and force signals from the first, second and third sets of straingauges; and determining the local pressure and the tangential forcesapplied to the upper contact surface of each of the one or morecantilever force pins.